This invention relates to a dry etching method employed in such applications as production of semiconductor devices, and particularly to a dry etching method for etching a silicon compound layer which method exhibits high selectivity to a resist, high selectivity to a silicon underlying layer, high etch rate, little damage, and low pollution.
The recent trend toward larger-scale integration and higher performance of such semiconductor devices as VLSIs and ULSTs is becoming technically demanding of dry etching methods for etching a silicon compound layer represented by silicon oxide (SiO.sub.2).
First, the mainstream of dry etching devices is transferring from the conventional batch processing to the single wafer processing for the following reasons: larger-scale integration expands the area of device chips, thereby increasing the diameter of wafers; patterns to be formed are made highly minute, so that uniform processing of the inside of wafer surface is required; and production with multiple varieties and small amounts as represented by ASIC is required. For maintaining productivity at a level similar to the conventional one with the single wafer processing, the etchrate per a wafer must be improved greatly.
Also, if impurity diffusion regions are shallow in the connection depth for attempting to achieve a high etchrate and minuteness, and if each variety of material layer is thin, an etching technique exhibiting higher selectivity to the underlying layer and less damage than the conventional technique is required. For instance, when contacts are formed in impurity diffusion regions formed in a semiconductor substrate, or source and drain regions of a PMOS transistor used as a resistive load element of SRAM, an SiO.sub.2 interlayer insulation film is etched by using a silicon substrate or a polysilicon layer as the underlying layer.
Further, improvement of the selectivity to the resist is also an important task. This is because, in a submicron device, generation of even a little dimensional transformation difference due to retreat of the resist may not be allowed.
Conventionally, the etching of the SiO.sub.2 based material layer has been carried out in a mode with high ionicity for cutting a rigid Si-O bond. Etching gases are typified by CHF.sub.3 and CF.sub.4, and the energy of incident ions of CF.sub.x.sup.+ released therefrom is used. However, for carrying out high-rate etching, it is necessary to increase the energy of incident ions, so that an etching reaction becomes proximate to a physical sputtering reaction. For this reason, the high etchrate and selectivity were constantly incompatible with each other.
Thus, usually, H.sub.2 or a depositional hydrocarbon based gas is added to the etching gas, so as to increase the apparent C/F ratio (ratio of the number of carbon atoms to the number of fluorine atoms) in the etching reaction system. By thus promoting deposition of carbonaceous polymer generated competitively with the etching reaction, high selectivity is achieved.
The present applicant has proposed a dry etching method for etching a silicon compound layer using, instead of these conventional etching gases, a saturated or unsaturated high-order chain fluorocarbon based gas with the number of carbon being two or more, in the Japanese Patent KOKAI KOHO (Publication of Unexamined Patent Application) Ser. No. 3-276626. This is aimed at achieving high-rate etching by efficiently forming CF.sub.x.sup.+ by using a fluorocarbon based gas such as C.sub.2 F.sub.6, C.sub.3 F.sub.8, C.sub.4 F.sub.10, and C.sub.4 F.sub.8. However, if only the high-order chain fluorocarbon based gas is used singly, the amount of F.sup.* to be formed increases, and therefore, the selective ratio to the resist and the selective ratio to the underlying layer cannot be sufficiently large. For instance, when an SiO.sub.2 layer on a silicon substrate is etched by using C.sub.3 F.sub.8 as an etching gas, though a high etchrate may be achieved, the selective ratio to the resist is as low as about 1.3. Therefore, etching durability is insufficient, and a dimensional transformation difference is generated by retreat of a pattern edge. Also, since the selective ratio to silicon is approximately 4.2, problems remain in overetching durability as well.
Thus, in order to solve these problems, two-stage etching is carried out in the prior art, wherein etching by singly using the high-order chain fluorocarbon based gas is stopped slightly before the underlying layer is exposed, and then the remaining portion of the silicon compound layer is etched by adding a hydrocarbon based gas such as ethylene (C.sub.2 H.sub.4) to the above-mentioned compound so as to promote deposition of carbonaceous polymer. This is aimed at increasing the apparent C/F ratio by supplying C atoms in the etching system and by consuming excessive F.sup.* with H.sup.* formed in a plasma so as to obtain HF.
However, under the status quo wherein design rules of semiconductor devices are becoming highly minute, the dimensional transformation difference from the etching mask is becoming hardly permitted. Even though the above-mentioned two-stage etching is carried out, it is necessary to improve further the selective ratio in etching on the first stage. In addition, it is anticipated that as the design rules of semiconductor devices become further minute from now on, the effects of particle pollution due to the carbonaceous polymer become serious. Therefore, it is desirable that the amount of the depositional gas such as a hydrocarbon based gas in etching on the second stage be reduced.
In view of such a status of the art, the present inventor has proposed a technique for etching a silicon compound by using an unsaturated chain fluorocarbon compound having at least one unsaturated bond in a molecule in a state in which the temperature of a substrate to be processed is controlled at 50.degree. C. or lower, see the Japanese Patent KOKAI KOHO Ser. No. 4-170026. The above-mentioned unsaturated chain fluorocarbon compound is exemplified by octafluorobutene (C.sub.4 F.sub.8) and hexafluoropropene (C.sub.3 F.sub.6). Since, theoretically, these gases form two or more units of CF.sub.x.sup.+ from one molecule on dissociation due to electric discharges, SiO.sub.2 may be etched at a high etchrate. Also, since the unsaturated bond exists in the molecule, it is easy to form highly active radicals by dissociation, whereby polymerization of the carbonaceous polymer may be promoted. In addition, deposition of the carbonaceous polymer may be promoted by controlling the temperature of the substrate being processed to be at 50.degree. C. or lower.
With this technique, the selectivity to the resist and the selectivity to the silicon underlying layer were improved significantly, and the particle pollution was reduced as well.
As is described above, the dry etching method using the unsaturated chain fluorocarbon compound which was previously proposed by the present inventor had extremely great advantages compared with conventional techniques. However, since these advantages are based mainly on improvement of the selectivity, there still remains the condition of particle pollution to be improved. In short, since this technique is not different at all from the conventional ones in terms of the mechanism for securing the selective ratio which is achieved by the deposition of the carbonaceous polymer proceeding competitively with the etching reaction, if the number of processing steps becomes large, the carbonaceous polymer will be deposited in the etching chamber, and the particle level will deteriorate. Accordingly, even though the particle pollution is reduced, it is merely such improvement as a reduction in frequency of maintenance for cleaning the etching chamber.